A liquid-pressure motor is typically used as a revolving motor or traveling motor for a revolving super structure of a construction machinery vehicle or the like, and is driven by a liquid-pressure circuit. A relief valve is used for liquid-pressure control of the liquid-pressure circuit.
One example of conventional relief valves is shown in FIG. 6 (see PTL 1, for example). A relief valve R10 will be explained in reference to FIGS. 6 and 2. A dashed line in FIG. 2 shows that liquid pressure (relief pressure) of an inflow opening 102a of the relief valve R10 changes with time.
Currently, the relief valve R10 is in a state shown in FIG. 6, and each of the pressure of the inflow opening 102a formed at a valve seat 102 and the pressure of an outflow opening 101a is tank pressure PT (state (1) shown in FIG. 2).
Next, for example, when the inflow opening 102a is rapidly pressurized (state (5) shown in FIG. 2), the liquid pressure of the inflow opening 102a is introduced to a third liquid chamber 140 through a restrictor 103b. Then, when the liquid pressure of the inflow opening 102a increases, and an upward force (rearward acting force) acting on a plunger 103 by the liquid pressure of the inflow opening 102a becomes higher than a downward force (forward acting force) acting on the plunger 103 by the liquid pressure of the third liquid chamber 140 and an elastic force (spring force) of a spring 105, the plunger 103 moves upward, that is, rearward, so that the inflow opening 102a and the outflow opening 101a communicate with each other. With this, a pressure liquid of the inflow opening 102a can be released through the outflow opening 101a at a predetermined flow rate from a state of initial set pressure P2.
Next, the liquid pressure of the third liquid chamber 140 is introduced to a first liquid chamber 122 and a second liquid chamber 132 through a communication hole 111 and restrictors 111a and 112 formed at the piston 104. Then, when the forward acting force applied to a forward acting surface 131 by the pressure liquid becomes higher than the total of a rearward acting force applied to bottom surfaces 123 and 124 of the third liquid chamber 140 and a rearward acting surface 121 by the pressure liquid and the elastic force of the spring 105, the piston 104 starts moving downward, that is, forward.
As above, when the piston 104 moves downward, that is, forward, the pressure liquid in the first liquid chamber 122 is discharged to the third liquid chamber 140 through the communication hole 111 and restrictor 111a of the piston 104. Therefore, the piston 104 slowly moves forward. Until the piston 104 moves forward as above to contact a step portion 101b of a case 101, the spring 105 is gradually compressed, and a repulsive force of the spring 105 gradually increases. Therefore, the relief pressure of the inflow opening 102a smoothly increases (state (6) shown in FIG. 2).
When the piston 104 moves forward to contact the step portion 101b of the case 101, the piston 104 cannot move downward, that is, forward any more. Therefore, the relief pressure is maintained at a constant maximum relief pressure PS (state (4) shown in FIG. 2). At the time of the states (6) and (4) shown in FIG. 2, the pressure liquid of the inflow opening 102a has pushed the relief valve open and is flowing out through the outflow opening 101a. Therefore, the pressure increase from the initial set pressure P2(t1) to the maximum relief pressure PS(t3) can be smoothly performed.